Spherical reflector with lens to convert to parabolic surface



EE RM C. E. woLcoTT 3,430,245

SPHERICL REFLECTOR WITH LENS TO CONVERT TO PARABOIJIC` SURFACE Feb. 25, 1969 Filed May 19. 1965 r agri/avr INVENTOR. Wf

y v// BY 3,430,245 SPHERICAL REFLECTGR WITH LENS 'I0 CONVERT` T PARABOLIC SURFACE Charles E. Wolcott, El Cajon, Calif., assigner to Whittaker Corporation, Los Angeles, Calif., a corporation of California Filed May 19, 1965, Ser. No. 456,941

U.S. Cl. 343-755 Int. Cl. H01q 19/.10, 1/42, .I5/24 5 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to improvements in ann tenna construction and is particularly useful in the obtainment of a reflecting surface for microwave frequencies in space environment.

Briefly, the construction described herein involves a spherical shell of elastic material with an inner surface portion of the shell being provided with a reflecting sur face over which a dielectric material is coated such that such dielectric coating in effect, changes the spherical re= fleeting surface to an equivalent parabolic reflecting surface. Such spherical shell is of a material which'-v is essentially transparent for electromagentic waves at the fre quency of operation of the antenna. Such material is pref= erably an elastic foam material as is also the dielectric coating whereby the composite antenna structure, so defined, is elastic and may be compacted to occupy a relatively small storage space when not in use land which may automatically self-expand (upon release of compactfing forces) to its operating condition.

It is therefore a general object of the present invention to provide an antenna construction of this general char acter.

A specific object of the present invention is to provide an antenna which may be packaged into a small volume for purposes of space transportation and yet which may be deployed in a space environment into la semi-rigid operable condition.

Another specific object of the present invention is to provide means and techniques whereby in effect a spheri cal reflecting antenna surface may be converted into a parabolic reflecting antenna surface, the same being lac-1 complished in general by providing various means of radiation phase front correction or retardation.

Another specific object of the present invention is to provide an antenna of this character which is enclosed in an expandable space radome structure such that solar environmental conditions may be controlled.

The features of the present inventionwhich are Ibe-s lieved to be novel are set forth with particularity in the -appended claims. This invention itself, both as toits organization and manner of operation, together with fur-= ther objects and advantages thereof, may be Ibest understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is essentially a transverse sectional view of an antenna embodying features of the present invention. FIGURE 2 demnstrates in enlarged form a portion ananas atented Feb.. 25, 1969 of the antenna shown in FIGURE l together with various mathematical relationships.

FIGURE 3 is a view similar to that illustrated in FIG-= URE 2 but of an alternative construction.

As illustrated `in FIG. l the antenna is essentially spheriE cal in outward appearance and is defined by a spherical shell 10 preferably of elastic material such as, for ex ample, foam material, such material being transparent at those frequencies of electromagnetic waves energy which is eitherreceived or transmitted by the composite antenna structure.

A portion of the inner surface of the shell 10 is coated with a metallized reflector surface 12, such surface defining a portion of a sphere of the same internal radius as the shell 10; and such reflecting surface is illustrated as extending between the points 14 and 16 in FIGURE l.

This reflecting surface 12 is a thin coating which may be applied as, for example, by metal spraying an internal portion of the shell 10 or by afllxing thereto as, for example, by cement, a thin layer of metal foil.

Coated on the reflector surface 12 is a dielectricy ma`= terial 18 which preferably flexible plastic foam material.

The purpose Afof this foam material 18 is to introduce a delay in the transmission of electromagnetic Wave en= ergy such that incremental portions of the reflecting sur= face 12, in effect, are displaced rearwardly to the left as indicated in FIG. 2 to form an equivalent reflecting' sur face 20 which is parabolic.

In accordance with the relations set forth also in FIG. 2 wherein R is the internal .radius of the shell 10 and also the radius of the reflector 12; the distance F is the `focal distance of the parabolic reflector 20 and is measured from the 0,0 coordinate 22 to the electromagnetic focal point 24 of the parabolic reflector 20. In the example illustrated, the ratio of F/D is equal to 0.35 where D is the diameter of the actual reflector 12. The equation for the parabola 20 may thus be defined by the equation x=v2/4F.

FIG. l illustrates also electronic and positioning equipment 30 supported by guy wires 32 an internal portion of the shell 10. The positioning equipment 30 serves to position and maintain the horn structure 34 at the focal point 24 in essentially a Newtonian type antenna arrange ment.

It will be evident to one skilled in the art that the antenna arrangement instead of being of the Newtonian type may be of any other particular desired type as, for example, of the Cassegrian type in which latter case a hyperbolic reflector may be supported internally of the shell 10 by guy wires of dielectric material and the antenna feed may be located also inside of the shell 10 in a Cassegrian configuration.

Because the effective reflecting surface is parabolic, i.e., transformed from a spherical reflecting surface into a parabolic reflecting surface, the wavefront from the antenna as indicated in FIG. l is perpendicular to the axis of the parabola.

This conversion of a spherical surface into a paraboloid is primarily electromagnetic in nature and is accom plished by the application of dielectric materials.

It is well known that a (dielectric material retards an electromagnetic wave passing through it, and has an index of refraction defined as the square root of the materials dielectric constant. This material causes a modification of phase. In other words the dielectric ma-l terial vretards the phase front of electromagnetic radia= tion in an amount depending upon its thickness. As illus trated in FIGS. l and 2, a central portion extending between the points 40 and 42 need not be covered with the dielectric material since in that central region the spherical reflector surface and the desired effective paraebolic reflector surface are essentially coincident. Indeed with a ratio of F/ D of 0.225, the parabolic curve and the curve of constant radius are coincident over substantially 60% of the radial y coordinate. This means that for a foot diameter sphere, a six foot diameter area of the sphere could be used as a parabolic reflector when the ratio of F /D is equal to 0.225c

This ratio decreases when the F/D ratio is increase to, for example, 035 as illustrated in FlG. 2

In FIG. 2 the dielectric material T13 is gradiently applied to the inner surface of' the reflector l2 and continuously increases in thickness as the spacing of the spherical reflector from the x-x axis increases in the y coordinate. The physical thickness of the dielectric d, is related to the electrical thickness, 1, in one way wave or ray traversal in accordance with the following equawhere s Using this equation and considering also the geometry of the spherical reflecting surface and also Snells law 'which defines the optical path through the material in terms of Vits refractive index, one is able to trace various rays so that they all have the same phase, ie., are delayed in the required amount at the time they reach the plane wavefront indicated in FIG 2C FIG 2 indicates that there is an apparent element of magnification in that the actual spherical reflector diameter is less than the equivalent parabolic reflector as described by the electromagnetic ray pathg This, how-1 ever, cannot be regarded as a gain since the projected aperture at a certain dissipation factor of the dielectric :material must be taken into considerationa Since the ray correcting refractive medium is a, dielectric material, it possesses some value of attenuation due to the dissipation factor which, in plastic foam and titanate materials, is usually quite small. The power lost by dissipation is considered to have only minor eifect 'upon antenna gain or directivityo In the arrangement shown in FIG. 2 the foam material used to impart a delay is a homogeneous material and hence its thickness increases, the material being thicker, the greater its displacement from the x-1x axis. Instead of using a homogeneous material the same may be loaded with an element which has a very high dielectric constant and low dissipation factor such as barium titanatee n this case the dielectric loading is such that the layer may be constant in thickness but yet there is a resultant di electric constant which is the product of the ratio of dielectrics per unit volume so as to obtain the same result as in FIGD 2, ie., a greater delay at locations further from the 9cm-x axisn This is illustrated in FIG 3 wherein the dielectric material is illustrated as being of constant thickness and with the same being loaded in different amounts with barium titanate as described abovea Many advantages result from the fact that the antenna spherical in shape and is of flexible materiaL The entire spherical radorne structure and the equivan lent parabolic reflector can be made flexible and there= fore collapsible for eicient packagingu When the packu aged antenna structure is released, the elastic memory of the materials expand the sphere into its original shape 'thus deploying the antenna into its operating conditiom To obtain full elastic recovery, a pressurization process may be used to smooth out any creasing and Wrinkling of the structural material which might have occurred when in its -folded and collapsed stateo Incefull memory of the spheres elastic material. has restored, the spherical structure maintains its spheri= all cal. geometry precisely in the space environment followa ing any perforation and depressurization by meteoritesu vUsing flexible plastic foam which has a very low dielec-v tric constant and dissipation factor, and also using flexible plastic laminate skins, which also have a relatively low dielectric constant dissipation factor', the wall of the spherical shell can be constructed to act as a radome 'which is of course very desirable in this application.

ln each of FGS. 2 and 3 the radiation retardirxg material is continuously gradated outwardly from the central portion of the actual reflecting surface so as to produce an. increasingly greater retardation to radiation the greater the perpendicular distance such retarding material is located from the central axis which passes through such central portion, In FIG 2 this gradation is accomplished by increasing the thickness and in FIG. 3 is accomplished by having the dielectric constant of the material increase progressively with increase in distance from. the x--x axis While there has been described what is at present con sidered the preferred embodiment of the invention, it will be understood that various changes and alterations may be made therein without departing from the essence of the invention and is intended to cover herein all such changes and alterations as come within the true scope and spirit of the annexed claims,l

I claim: y

l.. An' -antenna structure including a spherical shell of radiation transparent material; said spherical shell having an axis passing through its center; a spherically shaped radiation reflecting surface on an internal portion of said shell and having its central portion intersected by said axis and symmetrically disposed around said axis; said reflecting surface having an area which is not greater than one half of the internal surface area of said shell; radiation retarding material on said reflecting surface and cono tinuously gradated outwardly from said central portion and said axis to produce an increasingly greater retardaE tion to radiation the greater the perpendicular distance such retarding material is located from said axis.

2 An antenna structure as set forth in claim 1 in which said radiation transparent material and said radiation ren tarding material are elastically deformable to occupy a space substantially less than the volume of said'shell upon application of forces radially inwardly to said spherical shell and being capable of returning to its original form upon release of said forcesu 3. An antenna structure as set forth in claim ll in which said radiation retarding material is so gradated that it is substantially homogeneous and has an increasingly greater thickness the greater the perpendicular distance the same located from said axis.,

All, An antenna structure as set forth in claim i in which said radiation retarding material is so lgradated that it has substantially the same thickness but is non= homogeneous.

5o An antenna structure as set forth in claim l in which said material is so gradated that said spherically shaped radiation reflecting surface is transformed effectively into a paraboloid having its focal point lying on said axis,

References Cited 'UNITED STATES PATENTS 4./1955 Jaffe .N,s 343-755 OTHER REFERENCES EMLEBERMAN, Primary Examineru 

